The invention concerns processes for manufacturing hollow bodies from thermoplastic material in which a preform is first produced by injection before obtaining the final container in the course of a process that includes at least one blow-molding step.
The preform obtained by injection generally has a tubular, cylindrical body closed at one of its axial ends and which at its other end is extended by a neck, also tubular. The neck is generally injected so as to already have its final form when the body of the preform is required to undergo relatively substantial deformation to form the final container following the blow-molding operation.
More particularly, these processes concern the manufacture of containers made of polyethylene terephtalate (PET).
There are several variations of the procedures in this family. Depending on the process used, the corresponding manufacturing facility has one or more temperature condition ovens.
In all of the processes concerned, the blow-molding operation of the body of the preform requires that said preform be brought to a temperature that is higher than the glass transition temperature of the material. To that end, the preform is temperature conditioned by circulating it inside an oven. The oven has heating means which are, for example, comprised of infrared lamps in front of which the preform is moved by a conveyor device. Advantageously, provision is made so that the preform rotates while it is moving through the oven.
The preforms are generally held on the conveyor device by a gripping device formed by a mandrel that is engaged inside the neck of the preform and which holds it by tightening against the inner face of the neck. However, in some cases the preform is held by the outer face of the neck.
In some of these processes, the temperature resistance of the body of the final container is increased by increasing the rate of crystallinity of the PET. This is achieved by first putting the preform through a first blow-molding stage to obtain an intermediate container that is subjected to a heating stage in a temperature condition oven, during which stage it shrinks. This shrunk container undergoes a second blow-molding stage which gives the container its final form.
In other processes, it is the neck of the preform, of the final container or of an intermediate container, that is heated to increase the crystallinity. To that end, the facility has a suitable temperature condition oven.
In all of these cases, it can be seen that facilities that manufacture containers from thermoplastic material can have several types of temperature condition ovens, which ovens, of course, are suitable for the heat treatment to be carried out. However, all of these ovens may have the same type of conveyor system.
These different ovens, therefore, are intended to process objects in different phases of producing the final container. However, they are all intended to grip this object by the neck, which does not undergo any fundamental variation in its geometry, even if some processes result in a modification of the structure of the material of which the neck is composed. Also, in the text that follows the terms preform, blank or bottle are used indiscriminately to designate the object to be processed in the oven.
According to a known technique, the conveyor system can, for example, be comprised of a chain of conveyor links intended to carry a preform or a blank of the container in order to move it around a circuit inside the oven. This chain is driven in continuous movement.
To pick up the preform, a conveyor link has a gripping device that is axially movable with respect to the link and which, when it is taken from an extreme release position to an extreme gripping position, is axially engaged on the neck of the blank.
Different means are known for controlling the movements of the gripping devices between their released and gripping positions. Thus, in general a system of cams and cam followers is used to take advantage of the fact that, in general, the movement of the gripping devices is perpendicular to that of the conveyor chain. In a conventional manner, each gripping device is equipped with a roller which is intended to cooperate with sloping ramps that are integral with the infrastructure of the oven to cause the gripping device to raise or lower to the level of the preform loading or unloading stations.
However, in some cases the means for controlling the cams is not entirely satisfactory. This is especially the case when the gripping device has a mandrel that is intended to be inserted and very tightly secured inside the neck of the preform.
The loading and unloading of the preform, then, requires a relatively sizeable force applied along the axis of the gripping device. With a system of cams, such axial force becomes even greater, exerted on the conveyor chain in the direction of movement of said conveyor. Thus, each time a roller comes into contact with a cam, the chain jerks. This is harmful, first to the proper synchronization between the chain and the other components of the blow-molding facility, and second, to the working life of the conveyor system. This jerk becomes greater as the speed of movement of the gripping device increase, because then a cam with a steep slope will have to be used. In addition, in this case, the contact pressure between the roller and the cam becomes heavy to the point of threatening the working life of these two elements.
Moreover, in known embodiments, the roller is offset with respect to the axis of movement of the gripping device. Also, when the loading or unloading effort is large, the roller causes a sizeable rotational torque on the corresponding link of the chain, which can be the source of premature wear of the conveyor system.
A purpose of the invention, therefore, is to propose new means of controlling the movements of the gripping devices that make it possible to limit the forces on the conveyor chain even when the force in loading and unloading the preform is large.
To that end, the invention proposes a temperature condition oven for a blow-molding facility for producing containers of thermoplastic material, of the type with a conveyor chain of links having gripping devices each of which is intended to carry a blank of the container in order to move it around a circuit inside the oven, of the type in which the chain is driven in continuous movement along its circuit, of the type in which each gripping device is axially movable with respect to the link that carries it so that when it is taken from an extreme release position to an extreme gripping position it engages axially on the neck of the blank, and of the type in which it is provided with means to control the movements of the gripping devices between their release and gripping positions, characterized in that the means for controlling the movements of the gripping devices comprise at least one mechanism provided with an arm that is movable along a trajectory that is essentially parallel to the axial direction of gripping and which carries an interface device intended to come into contact with the gripping device, and in that the interface device is movably mounted on the arm in such a way as to allow non-slip contact with the gripping device while being immobilized with respect to the arm along the axial direction of gripping.
According to other characteristics of the invention:
the arm is controlled in its trajectory along a movement that alternates between an initial position and a final position; and, for a position of contact of the arm that falls between its initial and final positions, the interface device comes into contact with the gripping device which is in one of its extreme positions and forces it towards the other of its extreme positions, which position corresponds to the final position of the arm;
the interface device is mounted on the arm so that it is movable in a direction parallel to the movement of the links of the chain;
the interface device is elastically drawn back to a rest position with respect to the arm, and as a result of the movement of the corresponding chain link between the moment the arm comes into contact with the interface device and the moment the arm reaches its final position, the interface device is moved away from its rest position by the gripping device with which it is in contact;
between the contact position and the final position of the arm, the gripping device cooperates by adherence to the interface device to move it away from its rest position with respect to the arm;
during the return movement of the arm from its final position to its initial position, the interface device is drawn back to its rest position by elastic means;
the interface device is mounted on the arm slideably in the direction of movement of the links of the chain;
the interface device is comprised of at least one roller which is mounted on the arm rotatably around an axis perpendicular to the axial direction of gripping and to the direction of the movement of the links of the chain;
between the moment the interface device comes into contact with the gripping device and the moment the arm reaches its final position, the gripping device cooperates by adherence to the interface device to drive the roller in rotation around its axis;
the gripping device has a rod one central section of which is axially guided in the corresponding link and the two opposite ends of which cooperate respectively with the blank and with the mechanism, the mechanism exerting a pushing force on the rod, and the interface device cooperates by pressing against one surface of the rod that is essentially flat and perpendicular to the axial direction;
the gripping device has a rod one central section of which is axially guided in the corresponding link and the two opposite ends of which cooperate respectively with the device and with the mechanism, the mechanism exerts a pulling force on the rod, the interface device has the shape of a U-section having two parallel limbs, the ends of which are turned inward to delimit an aperture allowing the rod to pass through, the rod of the gripping devices has a control head which is intended to move between the two limbs of the U-section and which has a width that is greater than the width of the aperture in such a way that when the interface device is moved axially by the arm, the turned ends of the U-section cooperate with the head to control the axial movement of the gripping device;
the gripping device has a rod one central section of which is axially guided in the corresponding link and the two opposite ends of which cooperate respectively with the blank and with the mechanism, the mechanism exerts a pulling force on the rod; the arm has the shape of a U-section having two parallel limbs, the ends of which are turned inward to delimit an aperture allowing the passage of the rod; each of the two turned ends has at least a roller; the rod of the gripping devices carries a control head that is intended to move between the two limbs of the U-section and which has a width that is greater than the width of the aperture in such a way that when the arm is moved axially, the rollers of the turned ends of the U-section cooperate with the head to control the axial movement of the gripping device;
the alternating movement of the arm is controlled by a connecting rod and crank system; and
the means for controlling the movements of the gripping means comprise two mechanisms, one of which controls the movement of the gripping devices from their extreme release position to their extreme grip position, and the other controls the reverse movement.